]6g ON AERIAL NAVIGATION. 



sure of the velocity of the bird. The angle b d c can be 

 increased at tl e will of the bird, and to preserve a perfectly 

 horizontal path, without the wing being waved, must con- 

 tinually be increased in a complete ratio, (us; less at present 

 to enter into) till the motion is stopped altogether ; but at 

 one given time the position of the wings may be truly re- 

 presented by the angle bdc. Draw d e perpendicular to 

 the plane of the wings, produce the line e d as far as re- 

 quired, and from the point e, assumed at pleasure in the 

 line d e, let fall ef perpendicular to df. Then d e will 

 represent the whole force of the air under the wing; which 

 being resolved into the two forces efiwAfd, the former 

 represents the force that sustains the weight of the bird, the 

 latter the retarding force by which the velocity of the mo- 

 tion, producing the current c d, will continua'ly be dimi- 

 nished, ef is always a known quantity, being equal to the 

 •weight of die bird, and hence f d is also known, as it will 

 always bear the same proportion to the. weight of the bird, 

 as the sine of the angle b d e bears to its cosine the angles 

 def, and bdc, being equal. In addition to the retarding 

 force thus received is the direct resistance, which the bulk 

 of the bird opposes to the current. This is a matter to be 

 entered into separately from the principle now under consi- 

 deration ; and for the present may be wholly neglected, un- 

 der the supposition of its being balanced by a force precisely 

 equal ani opposite to itself. 

 Some practical Before it is possible to apply this hasisDf the principle of 

 observations, flying in birds to the purposes of aerial navigation, it will 

 be necessary to encumber it with a few practical observa-. 

 Problem. tions. The whole problem is confined within these limits,. 



viz. To make a surface support a given weight by the ap- 

 plication of power to the resistance of air. Magnitude is 

 F rimenfs the first question respecting the surface. Many experiments 

 on the resht- have been made upon the direct resistance of air, by Mr. 

 anceoftheair - Robins, Mr. Rouse, Mr. Edgeworth, Mr. Smeaton, and 

 others. The result of Mr. Smeutorfs experiments and ob- 

 servations was, that a surface of a square foot met with a 

 resistance of one pound, when it travelled perpendicularly 

 to itself through air at a velocity of 21 feet per second. I 

 hnye tried many experiments upo:i a large s^ale to ascer-. 



tui n 



